As with the GNOME XRandR configuration
method, setting Virtual to too large a
value may result in a loss of hardware
acceleration, and thus an inability to
use Compiz and its desktop effects.

When I use the GNOME monitor applet, or the Monitors configuration in the System menu, the default xrandr settings puts the second monitor to the right of the first, and, as I found with this bug, for most monitors this creates a virtual desktop larger than the maximum 2048 horizontal resolution needed for hardware acceleration on my netbook hardware.

So, it seems like if I can modify xrandr's default settings so that it places the new desktop above or below (north or south of) the main LVDS display, then hardware acceleration, and therefore compiz will continue to work. Can anyone tell me, what is the easiest way to achieve this?

UPDATE:

I have confirmed that multihead support with desktop effects and hardware acceleration works when I move the external monitor display north of the main LVDS display. Right now this involves the following process: plugging in the external monitor, starting the Monitors configuration menu, desktop effects are disabled automatically (and all of the windows on my workspaces are moved to the first workspace), repositioning the external display so that it is north of LVDS display and clicking apply, and then navigating to the Appearance menu and telling it to reenable desktop effects. Is there a simpler way do this?

UPDATE 2:

OK, so I thought that perhaps the GNOME Monitors configuration screen was trying to be clever, and might be disbling desktop effects. So, I just tried using the xrandr command-line client instead, as follows:

xrandr --output VGA1 --above LVDS1

When I do that, desktop effects are still disabled, and I need to manually reenable them. This, despite the fact that hardware acceleration works, and there is never a point where hardware acceleration stops working because the horizontal dimension of the virtual display is too large. So what program is trying to be clever, and is turning off desktop effects when it doesn't need to? And how do I make it stop?

If there were a way to re-enable desktop effects from the command line, which I could then put into a script along with the proper xrandr invocation, I would accept that as a workaround.

UPDATE 3:

OK, here's my script to enable a second monitor with desktop effects. It might be evil, I'm not sure:

3 Answers
3

Laying them out left-to-right is not the xrandr default, but the gnome-settings-daemon default. From gnome-settings-daemon 2.32 onwards, the last configuration should just be loaded from monitors.xml; so if you've done the vertical layout once, then next time it would automatically pick that up without kicking out Compiz.

So are you running 10.04 Lucid by any chance? In that case, you can follow these instructions to enable this feature in gnome-settings-daemon 2.30.1.

Or you can just get the compiz fusion icon, it allows you to easily click your way to reloading compiz. Thanks so much, had the same issues, never thought to reload compiz, just thought I was going to have to live with that in dual-mode. Just tried it with fusion icon, voila! Cube, everything normal again.